Transmitter hunting is a great excuse
for building your own equipment, and for experimenting with antennas and
electronic projects. But even if the local fire authorities have confiscated
your soldering iron, you can still come up with an effective set of transmitter
hunting equipment. Below, you will find some examples of very effective,
yet inexpensive, handheld radio direction finding tools you can build or
buy.

Note: an asterisk*
appears next to the heading of those tools that include their own receiver.

Beam,
Attenuator, & Receiver

A basic on-foot transmitter hunting set-up
consists of a beam antenna, an attenuator, and a receiver. No additional
equipment is required to successfully participate in most recreational
transmitter hunt events. Your choice of antenna, attenuator, and receiver
will depend on your budget, and your preference to build or buy.

For an easy-to-build, low-cost, high-performance
directional antenna, it is hard to beat the popular tape
measure yagi. Attach an offset
attenuator between the antenna and your receiver, and you've got the
complete on-foot transmitter hunt set shown below. I have been unable to
locate a source for offset attenuator kits, or assembled and tested offset
attenuators. Does anyone smell a business opportunity?

Buy a Handheld Beam

Would you prefer that someone else built
the antenna? Here are several sources of pre-built antennas. Note: you'll
still want an attenuator
to use with any of these beams.

Shoestring
Antennas (of Keyport, WA) offers a handheld 2-element quad antenna,
that is light weight, and collapses into a small bundle of wire and fiberglass
sticks that fits neatly into a nylon bag for storage. Best of all, it isn't
a kit. It comes ready to use, with only some minor assembly required. Their
2-element quad, with one of the attenuators mentioned above, will turn
your HT into a competitive transmitter hunting tool. Keep in mind, however,
that quad antennas are prone to snagging on tree limbs and low branches.
This quad is durable enough to survive the rough treatment, but extracting
an antenna from an obstinate bush does tend to slow one down.

Arrow
Antenna (of Cheyenne, WY) offers a couple of varieties of handheld
3-element yagis, including one that converts into a 4-element antenna.
The yagi elements are made from aluminum arrow shafts. They should be sturdy,
but not as forgiving as the
tape measure yagi's
elements.

WiMo
Antenna Ltd. (of Germany) offers an HB9CV
antenna that should be suitable for on-foot transmitter hunting. It can
be dismantled, and stored in a small pouch when not in use. The HB9CV is
a 2-element design in which both elements are fed off-center. The design
is commonly used in Europe for transmitter hunting.

Ron
Graham Electronics (of Australia) offers an HB9CV antenna designed
especially for on-foot hunting. The model ANT1/144
antenna is optimized for front-to-back ratio. It is compact and light
weight, and has flexible antenna elements that resist breakage. The antenna
comes ready to use, and only minor assembly is required.

A
Complete Kit from Down Under*

This kit from Ron
Graham Electronics (of Australia) was popular at the Region 2
ARDF Championships held the Summer of 1999 in Portland, Oregon. It consists
of two components that can be purchased separately: the model RX1 receiver,
and the model ANT1/144 antenna. You can find out more about this kit at
the web site above, or by sending e-mail to rongraham@magnet.com.au.

The model RX1
receiver has adequate sensitivity for most on-foot hunts, and a built-in
attenuator that eliminates the need for an external attenuator. The dual-conversion
AM design works also for FM signals, using slope detection. When paired
with the model ANT1/144 2-meter antenna (described above) the combination
makes a complete, compact, self-contained transmitter hunting tool. RX1
price: $99 Australian + shipping, ANT1/144 price: $46 Australian + shipping
(prices subject to change). Use a currency
converter to see what those Australian dollars amount to in your favorite
currency. Note: the receiver is not a suitable kit for a first-time builder,
however, it might be possible to purchase one fully assembled and tested.
Contact Ron Graham Electronics to inquire.

An Australian Sniffer*

In Australia, a receiver with a built-in
attenuator is called a sniffer. In the recent past some enterprising
Australian entrepreneurs have offered sniffer kits to fellow transmitter
hunters, with reportedly excellent results.

A complete, assembled, sniffer is available
from Bryan Ackerly (VK3YNG). The VK3YNG sniffer comes fully built and tested.
Just add batteries and antenna. Take a look at the user's manual here:
http://www.users.bigpond.net.au/vk3yng/foxhunt/2m_sniffer/manual.htm.
As of this writing (Dec 2002) this unit is available for purchase, but
don't wait, they tend to sell out quickly. Contact Bryan
Ackerly for payment and shipping details.

A Korean Competitor*

Agayon
Amateur Radio Research Group (of South Korea) has developed the AGAYON
model AR-2e, a competition-grade ARDF receiver/antenna combination.
Clearly, a lot of effort has gone into this design. I have no experience
with this device, and have not heard any reports on its performance. I
have repeatedly requested price information, and clarification on whether
this is a kit or a fully-assembled unit. So far I have never received an
answer to my inquiries. Please contact
me if you have any information on this device.

The Venerable Little L-Per*

L-Tronics
(of Santa Barbara, CA) has been selling its VHF and UHF radio direction
finding receivers for search and rescue since the 1970's. These units are
still available today. They are not often used by the recreational transmitter
hunting community for several reasons. Price is one obstacle: a new model
LH-20 covering the frequency range of 136-150 MHz costs $475 (price subject
to change). Also, the Little L-Per is rock bound: a crystal is required
for each hunt frequency. But if you have access to an L-Per, or have the
opportunity to purchase one at a bargain price, it might be worth making
the investment in some crystals for the usual transmitter hunt frequencies.
Consult reference [3] for a much more thorough
treatment of the Little L-Per.

Handy Finders

The term "handy finder" is often applied
to various time difference of arrival (TDOA) radio direction finders, such
as the HANDI-Finder® (reference [4]).
TDOA RDF units consist of two identical antennas, mounted to a rigid support,
and spaced less than 1/2 wavelength apart. An electronic switching mechanism
alternately connects one antenna, and then the other, to an attached FM
receiver, at a rate of several hundred times per second. The FM receiver
detects the induced phase shift in a received signal, and outputs an audible
tone. When both antennas are equidistant from the signal source, there
is no induced phase shift, and a null in the audio tone is observed. In
use, a handy finder's support is rotated until a null is observed, indicating
that the transmitter lies along a line perpendicular to the plane of the
antennas.

Handy finders are simple and inexpensive
to build. They also do not suffer from overload in high signal strength
conditions.

Unfortunately, handy finders have several
drawbacks. One serious drawback is the 180-degree ambiguity of bearing
measurements. Until one resolves the ambiguity, by moving a distance along
a path tangential to the signal wavefront, it is not possible to determine
whether one's direction of movement is toward, or away from, the transmitter
being hunted. Frustration and fatigue set in quickly when a significant
amount of time and effort is wasted moving sideways to the direction of
the transmitter.

As serious as the directional ambiguity,
is the handy finder's susceptibility to multipath signals. A raspy sounding
audio tone, with multiple nulls as the antennas are rotated, is a classic
indication of a reflected signal interfering with a TDOA unit. An experienced
handy finder operator will recognize the telltale tone, and will attempt
to find a better location for bearing measurements. Under the same signal
conditions, a hunter using a simple beam antenna might notice the peak
in signal strength in the direction of the reflected signal, but would
also observe the stronger peak in the direction of the transmitter, and
would know immediately in which direction to head. The handy finder hunter
is at a serious disadvantage.

The limitations of TDOA devices restricts
their usefulness to hunting within a hundred meters or so of a transmitter,
where a few steps to the side results in a noticeable change in bearing
(and body fade can also be used for
removing bearing ambiguity). At such close range a line-of-sight
signal is likely to overwhelm most reflections, helping neutralize the
handy finder's multipath vulnerability.

TDOA units that include narrow audio filters,
and circuits that determine left-or-right signal direction, are a step
above the simple handy finder. If you're thinking about building a TDOA
DF tool, consider this
design by Joe Leggio. Its synchronized sampling of the receiver audio,
and superior antenna switching design, should make it perform more dependably
than simpler designs.

The Bottom Line

Every tool for transmitter hunting has
its advantages and disadvantages. Do a little research and choose what
is best for you based on your budget, time, and skills. In the end, it
is those who know their equipment that fare the best. So, whatever you
use, practice with it ahead of time. Use tape, velcro, or cable ties to
hold things together. Remember: you'll be walking around with this equipment,
and you may have a map, or other tools to carry as well.